1. Field of the Invention
The present invention relates to a thermosensitive recording material which reduces curling toward a print surface before and after printing, concerning a sheet-like thermosensitive recording medium for a thermosensitive printer, which utilizes color-developing reaction between an electron-donating color-forming compound and an electron-accepting compound; and the present invention relates specifically to a thermosensitive recording material for medical images.
2. Description of the Related Art
A thermosensitive recording medium generally includes a support made of paper, synthetic paper, plastic film or the like, and a thermosensitive color-developing layer which is provided on one surface of the support and which contains as main components a colorless or pale color-forming material such as an electron-donating leuco dye, an organic acid developer such as an electron-accepting phenolic compound, and a binder; and it is possible to obtain a color-developed recorded image by making the color-forming dye and the developer react together, with the utilization of thermal energy, pressure, etc. Recording materials like this have been proposed in many varieties, and thermosensitive recording materials among them are advantageous in that complicated processes such as development and fixation are not required, recording is made possible in a short period of time with a relatively simple apparatus, noise does not arise much, and the costs are low, for example; accordingly, the thermosensitive recording materials are widely used as recording materials for electronic calculators, facsimiles, ticketing machines, label printers, printers for CRT medical measurement, CAD printers, recorders, scientific measurers, plotters and the like.
Each of these thermosensitive recording materials is generally produced by applying the color-forming dye and the developer onto paper. As a system that meets such a requirement, there is a thermosensitive recording process with a thermosensitive recording material; since the thermosensitive recording material for such use is required to have printing uniformity and stiffness comparable to that of a conventional silver halide X-ray film and be free of paper powder or the like which may lead to a printing defect, the thermosensitive recording material has been produced by applying the color-forming dye and the developer not onto paper but onto biaxially-stretched synthetic paper composed mainly of a plastic support, particularly polypropylene.
As to such a thermosensitive recording material with a plastic support, since the heat resistance of the support itself is low in comparison with that of a support made of paper, there is a problem in which printing by a thermal head causes the plastic support to contract and thus a film lifts, namely curls, toward the print surface. Before printing as well, there is a problem that when a thermosensitive recording layer and a protective layer are applied onto the support and dried, the film curls toward the surface because of expansion and contraction of the support and the applied layers. Curling caused by expansion and contraction of the applied layers is particularly conspicuous in the case of a thermosensitive recording medium which is provided with a layer composed mainly of a high Tg resin for the purpose of yielding a high surface glossiness of 60% or greater. As methods for reducing the curling, the following applications have been published.
Japanese Patent Application Laid-Open (JP-A) Nos. 2004-284089 and 10-181205 each disclose a method for reducing curling by using an acrylic resin for the inside of a back layer. JP-A Nos. 2003-276330 and 06-239019 each disclose a method for reducing curling by using a core-shell acrylic resin for the inside of a back layer. However, these methods are insufficient to prevent curling caused by contraction of a support, which occurs especially when a synthetic paper support is used, as described above. Moreover, there are such problems that the back layer has less adhesiveness to the support than it should, and that applied layers easily crack.
Meanwhile, JP-A No. 2006-168319 describes a method for improving water resistance by adding a specific maleic acid resin into a back layer; however, the addition of only the specific maleic acid resin is insufficient to prevent curling caused by contraction of a support, which occurs especially when a synthetic paper support is used, as described above.
Further, Japanese Patent (JP-B) No. 3161774 describes a thermosensitive recording material in the form of a roll, in which a carboxylic acid-modified polyvinyl alcohol and polyamide epihalohydrin are used for an overcoat layer, and polyvinyl alcohol and an aziridine compound are used for a back layer. JP-B No. 3616839 describes a thermosensitive recording material in which a core-shell emulsion resin and an aziridine compound are used for a back layer. However, the provision of the back layer containing polyvinyl alcohol and an aziridine compound or the back layer containing a core-shell emulsion resin and an aziridine compound is insufficient to prevent curling caused by contraction of a support, which occurs especially when a synthetic paper support is used, as described above.
Among these thermosensitive recording materials, thermosensitive paper in which multilayered synthetic paper is provided as a support is used in the following cases: the case where water resistance and tensile strength are required; the case where used in an image printer for CRT medical measurement, which requires uniformity and high resolution of recorded images; and the case where used in a CAD plotter, which requires dimensional stability and thin line recording.
In the field of medical treatment, internal body conditions able to be viewed using X-rays, MRI, CT scans, etc. have been made into visible images on silver halide film, and those images have been visually observed for diagnosis and referred to by means of backlight employed in the film viewing method. However, the wet process for the silver halide film presents a problem with waste liquid disposal; further, along with the recent digitization of images, emergence of a dry process to take the place of it has been demanded, and there have already been cases where a thermosensitive recording system is put to medical use and designed for reference in diagnosis with a monitor that displays digital images, or for diagnosis by visual observation with output digital images, as well as for CRT medical measurement.
Thermosensitive recording materials for medical use are generally classified into the reflection type in which recording material has little or no light transmittance as a whole, and a formed image is viewed by means of reflection of light; and the transmissive type in which recording material has light transmittance as a whole, and the light transmittance is utilized. The present invention concerns the reflection type in which a formed image is viewed by means of reflection of light.
The properties required for an image on thermosensitive paper to be used as a reflection-type medical image for reference or diagnosis by visual observation are as follows: uniformity, high resolution, thin line recording capability, high glossiness, water resistance, curl reducing capability, dimensional stability and tensile strength of the recorded image. Accordingly, multilayered synthetic paper is provided as a support in the thermosensitive paper.
However, multilayered synthetic paper serving as a support in thermosensitive paper used for an electronic calculator, a facsimile, an automated ticketing machine, a scientific measurer, a CAD printer, a plotter, etc. is provided with depressions and protrusions on a surface thereof to improve printing suitability and writing capability as necessary properties, and some of the protrusions have heights unsuitable for a support of a thermosensitive recording medium for reflection-type medical images. When a medical image is recorded onto thermosensitive recording paper which includes such a support, there is, for example, a problem that white spots are formed at a halftone portion and a solid image recording portion, thus leading to a decrease in uniformity. Also, the provision of the depressions and the protrusions on the surface of the synthetic paper causes reduction in glossiness, and thus there is a problem that the high glossiness required for reflection-type medical thermosensitive paper to show a photograph-like image cannot be yielded. Moreover, the surface of the multilayered synthetic paper is provided with the depressions and the protrusions, whereas the other surface thereof has a different structure; thus, when the multilayered synthetic paper is formed into a sheet, the degree of curling is great, which is problematic in the case where a medical image is observed.
In order to remove the white spots formed at the halftone portion and the solid image recording portion, JP-A No. 03-190787 proposes and puts into practice a thermosensitive recording paper including a support, and a thermosensitive color-developing layer provided on the support, wherein the support is a synthetic paper composed of films in which a biaxially-stretched resin film serves as a base layer, and a uniaxially-stretched film made of a thermoplastic resin containing 10% by weight to 50% by weight of calcium carbonate powder is provided as a paper-like layer on a surface of the base layer, and wherein the support has the following properties (i) to (iii): (i) the opacity measured in accordance with JIS P-8138 is 45% or less; (ii) the Bekk smoothness of the paper-like layer onto which the thermosensitive color-developing layer is applied is 100 sec to 300 sec, and the center line average roughness (Ra) thereof is 1.5 μm or less; (iii) the density of the support measured in accordance with JIS P-8118 is 1.1 g/cm3 or less.
Meanwhile, JP-A No. 07-81231 proposes a thermosensitive recording paper including a support, and a thermosensitive color-developing layer provided on one surface of the support, wherein the support is a synthetic paper composed of films in which a biaxially-stretched resin film serves as a base layer, a paper-like layer formed of a uniaxially-stretched film made of a thermoplastic resin containing 1% by weight to 8% by weight of calcium carbonate powder is provided on one surface of the base layer, and a back surface layer formed of a uniaxially-stretched film made of a thermoplastic resin containing 15% by weight to 55% by weight of fine inorganic powder is provided on the other surface of the base layer, and wherein the support has the following properties (i) to (iv): (i) the opacity measured in accordance with JIS P-8138 is 45% or less; (ii) the Bekk smoothness of the paper-like layer onto which the thermosensitive color-developing layer is applied is 1,000 sec to 3,500 sec, and the center line average roughness (Ra) thereof is 0.5 μm or less; (iii) the Bekk smoothness of the back surface layer is 100 sec to 900 sec, and the center line average roughness (Ra) thereof is 0.6 μm to 1 μm; (iv) the density of the support measured in accordance with JIS P-8118 is 0.91 g/cm3 to 1.1 g/cm3.
These supports make it possible to somewhat rectify, for example, the problem that protrusions having unsuitable heights cause white spots to be formed at a halftone portion and a solid image recording portion and thus there is a decrease in uniformity; however, they do not fully satisfy the requirements for reflection-type medical images, and the high image glossiness required cannot be yielded either. Moreover, since the front and back of the base layer have different structures, the degree of curling is great.